Chapter 7: Instinct

Science is fun. Now, I know that someone telling you a thing is fun is usually a guarantee that it isn't. And I know that people who tell you science is fun usually do so in strained and pleading tones, and expect you to believe them because they have spiky hair and can play the harmonica. But it's true.

To see what I mean, read chapter 6 of the Origin, 'Instinct'. Despite, or because of, having its share of 'are you sure about that?' moments, it's a delight, because it shows Darwin doing the most fun thing in science: mucking about with reality, sometimes called experimentation.

In this chapter, Darwin brings behaviour into his theory. His reasoning exactly parallels his earlier reasoning on what he calls "corporeal structure". Behaviour is variable, it is heritable, and it is crucial. Ergo, it must be subject to natural selection.

Darwin is particularly interested in innate behaviours, those that show themselves regardless of environment or experience. You can see why: as such, they can only be explained through natural selection.

He allows for learning, but treats it as a form of behavioural Lamarckism, as a source of heritable behaviours that arise through "habit". But, at least for now, I have decided to stop worrying about Darwin's occasional bursts of evolutionary pluralism. Given how little anyone knew about biology, the point is that he discovered natural selection, not that he exterminated or accommodated Lamarckism.

To make his point, Darwin gives three case studies: cuckoos, slave-making ants, and the hive building of honeybees. These, he says, are the sternest tests he can think of. Explain these, he says, and we ought to believe that natural selection can indeed explain instinct.

First, cuckoos. The female cuckoo isn't mean, Darwin explains. She's inconvenienced.

"It is now commonly admitted that the more immediate and final cause of the cuckoo's instinct is, that she lays her eggs, not daily, but at intervals of two or three days; so that, if she were to make her own nest and sit on her own eggs, those first laid would have to be left for some time unincubated, or there would be eggs and young birds of different ages in the same nest. If this were the case, the process of laying and hatching might be inconveniently long, more especially as she has to migrate at a very early period[.]"

In this picture, the bird is less the ruthless parasite than someone caught with a stomach upset in central London, scurrying from one public toilet to the next. Darwin, as we've seen, didn't underestimate the power of selection or the severity of the struggle for existence. But he seems to have thought that brood parasitism was an adaptation to infrequent egg-laying, and not vice versa. This seemed even odder when I got to the chapter's last sentence, in which he mentions the "young cuckoo ejecting its foster brothers". So he knew that cuckoo chicks weren't ideal house guests.

That said, Darwin can see the advantage of having another bird raise your chick, and he uses it to explain the evolution of parasitism from a non-parasitic ancestor -- a bird a bit like the non-parasitic American cuckoo.

"[L]et us suppose that the ancient progenitor of our European cuckoo had the habits of the American cuckoo; but that occasionally she laid an egg in another bird's nest. If the old bird profited by this occasional habit, or if the young were made more vigorous by advantage having been taken of the mistaken maternal instinct of another bird, than by their own mother's care... then the old birds or the fostered young would gain an advantage."

There's a game I've been playing as I've read called 'What would Darwin have made of...?' WWDHMO the work by Nick Davies and his colleagues, revealing how deep the cuckoo's instinct goes? Different races of females are specialized to parasitize different species, their eggs mimic those of their hosts, and their chicks can beg at the same rate as a whole brood of their host chicks. And the hosts do everything they can to identify and eject the intruder. Unfortunately, it's quite a boring game, because usually the answer is 'He'd have been delighted'.

Next up, slave-making ants: those whose workers are stolen from other nests. Darwin spent three summers with his nose to the ground of Surrey and Sussex, tracking raiding parties, and experimenting with the ants:

"[M]y attention was struck by about a score of the slave-makers haunting the same spot... they approached and were vigorously repulsed by an independent community of the slave species (F[ormica]. fusca); sometimes as many as three of these ants clinging to the legs of the slave-making F. sanguinea. The latter ruthlessly killed their small opponents, and carried their dead bodies as food to their nest, twenty-nine yards distant; but they were prevented from getting any pupae to rear as slaves. I then dug up a small parcel of the pupae of F. fusca from another nest, and put them down on a bare spot near the place of combat; they were eagerly seized, and carried off by the tyrants, who perhaps fancied that, after all, they had been victorious in their late combat."

This is one of the most enjoyable passages in the book so far. I had a picture of readers across southern England lowering their copies and saying: 'So that's the lunatic we tripped over on our picnic.'

But hang on a minute. Tyrants? Darwin, as we have seen, could revel in nature's cut and thrust. Only in the previous chapter he described infanticide in bees in admiring terms. But far from admiring the resourcefulness and efficiency of these slave-making ants he describes the behaviour as "odious".

I couldn't say that this was the first time in the book he'd said something disapproving about something in nature, but it's certainly very rare. And I'm sure this wouldn't have struck me so much had I not been aware of Adrian Desmond and Jim Moore's new book arguing that Darwin's opposition to slavery drove his view on human origins.

So as to avoid writing a 3,000-word post, I shall largely skip over Darwin's experiments on the honeybee's ability to build a comb of hexagonal cells. His aim is to show that this complex structure -- "the most wonderful of all known instincts" -- was an emergent property of individual workers following simple rules. And also to shows that there was a continuum from crude to sophisticated cell-building in different bee species. This was one of several occasions where I wished the Origin had a few more diagrams.

To natural theologians, the geometrical perfection of the bee cell was one of the key pieces of evidence for design in nature. But post-Darwin, the major problem with social insects became not their technology, but their polity: the existence of sterile, altruistic workers.

As a social-insects geek, one of the quotations in the Origin that I've seen most repeated is this:

"[I] will confine myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to my whole theory. I allude to the neuters or sterile females in insect-communities: for these neuters often differ widely in instinct and in structure from both the males and fertile females, and yet, from being sterile, they cannot propagate their kind."

The story that I was taught goes a bit like this: Darwin shrugs his shoulders and moves on. In the 1930s, JBS Haldane shows he's cracked the problem when he makes an off-hand remark about laying down his life for two brothers or eight cousins, and then Bill Hamilton comes along in the 1960s and sorts everything out in mathematical terms, with the theory of kin selection, showing that genes can spread if they benefit their carriers' relatives.

But now that I've finally read Darwin, I think it's more complicated than that. For a start, he seems to come within a whisker of clearing up the whole matter immediately:

"This difficulty, though appearing insuperable, is lessened, or, as I believe, disappears, when it is remembered that selection may be applied to the family, as well as to the individual, and may thus gain the desired end. ... Thus I believe it has been with social insects: a slight modification of structure, or instinct, correlated with the sterile condition of certain members of the community, has been advantageous to the community: consequently the fertile males and females of the same community flourished, and transmitted to their fertile offspring a tendency to produce sterile members having the same modification."

If, like Darwin, you didn't know about genetics, and thought that inheritance was a process of blending, it's difficult to see how you could have made any more progress along this line of thought than he does here.

But I was also surprised to see that it's not actually sterility that bothers him most. This he dismisses in a few sentences, as no more difficult to explain than "any other striking modification". He spends far longer trying to explain how the large anatomical differences between the different castes (e.g. worker, queen, soldier) could have arisen by gradual selection acting on small variations, when the bearers of these traits don't get to pass them on.

In other words, he seems more interested in the 'how?' question than the 'why?' -- what's in it for the workers? 'Kin benefits' say most social-insect researchers, at least on my side of the atlantic, such as Andrew Bourke, Laurent Keller and Francis Ratnieks. 'Group benefits' say Bert Holldobler and E. O. Wilson in The Superorganism.

Marek Kohn's recent Nature article updates the debate between kin and group selection better than I can here. Instead, I had better get cracking and read the next chapter, Hybridism. I'll probably post on Thursday, but shall get back on schedule soon.